Conventionally, in the computer network, packet transmission and reception are carried out using a communication system called a packet communication system. Recently, demand for establishing a network using wireless communication is increasing in a household LAN (Local Area Network), for example. As compared with a wired LAN, the wireless LAN has such advantages that the installation of a wire such as a cable is not necessary and that the degree of freedom of moving a terminal connected to the LAN increases.
The IEEE802.11 radio communication system (a system based on the ANSI/IEEE Std 802.11, 1999 Edition) is present as a standard wireless LAN. The standard of the IEEE802.11 is further divided into details according to frequencies band and communication speeds used.
In the network such as the wireless LAN, plural communication devices connected to the network share one network route in time sharing, regarding transmission and reception of packets. The efficiency of band utilization changes greatly depending on managing methods of transmission right.
For example, when carrying out transmission and reception of motion data such as video data in streaming via the wireless LAN, occurrence of frame dropping, fuzzy images, and voice interruption is considered at the reception side, unless the transmission right is managed accurately. Accordingly, the IEEE802.11e is proposed as the standard which takes QoS (Quality Of Service) into consideration (see, for example, Nonpatent Literature 1).
The IEEE802.11e prescribes provision of a control station that manages the transmission right, within a specific communication network. Within the communication network, the control station carries out scheduling to give the transmission right to each transmitting station, considering transmission requests from plural transmitting stations that are going to transmit data. The control station transmits a packet called Qos CF-PLL, indicating provision of the transmission right, to each transmitting station, based on this scheduling. The transmitting station is permitted to transmit data only when the transmitting station is given the Qos CF-POLL from the control station, and can transmit the data only during a period called TXOP indicated by Qos CF-POLL.
As described above, because each transmitting station is given the transmission right following the scheduling of the control station, the transmitting station can use a communication band more properly corresponding to urgency of data transmission.
In the above communication network, when the transmitting station transmits data to the receiving station, the receiving station sometimes cannot properly receive the transmitted data for some reason. This problem becomes notable in wireless communication. Therefore, when the transmitting station transmits data to the receiving station, and also when the receiving station accurately receives this data, the receiving station transmits transmission confirmation information to the transmitting station. Upon receiving the transmission confirmation information, the transmitting station specifies a packet of which transmission has been unsuccessful, from the content of the transmission confirmation information. The transmitting station retransmits this packet. With this process, all packets are substantially securely transmitted to the receiving station.
The IEEE802.11e employs a system of using Block ACK for acknowledging plural packets, in addition to conventional Normal ACK for acknowledging individual packets, as kinds of ACK, to decrease the number of occupied bands. Specifically, when the transmitting station transmits a packet called Block ACK Request to the receiving station, and also when the receiving station receives this packet, the receiving station transmits to the transmitting station a packet called Block ACK containing a bit map indicating the transmission confirmation of a predetermined number of packets that the receiving station has received from the transmitting station. The transmitting station retransmits the packets based on the transmission confirmation information indicated by Block ACK. In general, the use of Block ACK improves the using efficiency of bands, as compared with when the conventional Normal ACK is used.
Nonpatent Literature 1: http://www.ili-info.com/ieee802drafts/2005.3.31